Sheeting with composite image that floats

ABSTRACT

Translucent, transparent, or semi-translucent microlens sheetings with composite images are disclosed, in which a composite image floats above or below the sheeting, or both. The composite image may be two-dimensional or three-dimensional. The sheeting may have at least one layer of material having a surface of microlenses that form one or more images at positions internal to the layer of material, at least one of the images being a partially complete image. Additional layers, such as retroreflective, translucent, transparent, or optical structure layers may also be incorporated into the sheeting.

This application is a continuation of U.S. patent application Ser. No.13/248,795, now U.S. Patent Publ. No. 2012/0019607, filed Sep. 29, 2011,which is a continuation of U.S. patent application Ser. No. 11/960,837,now U.S. Pat. No. 8,057,980, filed Dec. 20, 2007, which is acontinuation of U.S. patent application Ser. No. 11/399,695, now U.S.Pat. No. 7,336,422, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/898,580, filed Jul. 3, 2001, now U.S. Pat. No.7,068,434, which is a continuation-in-part of U.S. patent applicationSer. No. 09/510,428, filed Feb. 22, 2000, now U.S. Pat. No. 6,288,842,the entire contents of which are hereby incorporated herein by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to sheeting that provides one or morecomposite images that are perceived by an observer to be suspended inspace relative to the sheeting, and in which the perspective of thecomposite image changes with the viewing angle.

BACKGROUND OF THE INVENTION

Sheeting materials having a graphic image or other mark have been widelyused, particularly as labels for authenticating an article or document.For example, sheetings such as those described in U.S. Pat. Nos.3,154,872; 3,801,183; 4,082,426; and 4,099,838 have been used asvalidation stickers for vehicle license plates, and as security filmsfor driver's licenses, government documents, tape cassettes, playingcards, beverage containers, and the like. Other uses include graphicsapplications for identification purposes such as on police, fire orother emergency vehicles, in advertising and promotional displays and asdistinctive labels to provide brand enhancement.

Another form of imaged sheeting is disclosed in U.S. Pat. No. 4,200,875(Galanos). Galanos discloses the use of a particularly “high-gainretroreflective sheeting of the exposed-lens type,” in which images areformed by laser irradiation of the sheeting through a mask or pattern.That sheeting comprises a plurality of transparent glass microspherespartially embedded in a binder layer and partially exposed above thebinder layer, with a metal reflective layer coated on the embeddedsurface of each of the plurality of microspheres. The binder layercontains carbon black, which is said to minimize any stray light thatimpinges on the sheeting while it is being imaged. The energy of thelaser beam is further concentrated by the focusing effect of themicrolenses embedded in the binder layer.

The images formed in the retroreflective sheeting of Galanos can beviewed if, and only if, the sheeting is viewed from the same angle atwhich the laser irradiation was directed at the sheeting. That means, indifferent terms, that the image is only viewable over a very limitedobservation angle. For that and other reasons, there has been a desireto improve certain properties of such a sheeting.

As early as 1908, Gabriel Lippman invented a method for producing a truethree-dimensional image of a scene in lenticular media having one ormore photosensitive layers. That process, known as integral photography,is also described in De Montebello, “Processing and Display ofThree-Dimensional Data II” in Proceedings of SPIE, San Diego, 1984. InLippman's method, a photographic plate is exposed through an array oflenses (or “lenslets”), so that each lenslet of the array transmits aminiature image of the scene being reproduced, as seen from theperspective of the point of the sheet occupied by that lenslet, to thephotosensitive layers on a photographic plate. After the photographicplate has been developed, an observer looking at the composite image onthe plate through the lenslet array sees a three-dimensionalrepresentation of the scene photographed. The image may be in black andwhite or in color, depending on the photosensitive materials used.

Because the image formed by the lenslets during exposure of the platehas undergone only a single inversion of each miniature image, thethree-dimensional representation produced is pseudoscopic. That is, theperceived depth of the image is inverted so that the object appears“inside out.” This is a major disadvantage, because to correct the imageit is necessary to achieve two optical inversions. These methods arecomplex, involving multiple exposures with a single camera, or multiplecameras, or multi-lens cameras, to record a plurality of views of thesame object, and require extremely accurate registration of multipleimages to provide a single three-dimensional image. Further, any methodthat relies on a conventional camera requires the presence of a realobject before the camera. This further renders that method ill-adaptedfor producing three-dimensional images of a virtual object (meaning anobject that exists in effect, but not in fact). A further disadvantageof integral photography is that the composite image must be illuminatedfrom the viewing side to form a real image that may be viewed.

SUMMARY OF THE INVENTION

The present invention provides a microlens sheeting having a compositeimage that appears to be suspended above or below the sheeting. Thesesuspended images are referred to for convenience as floating images, andthey can be located above or below the sheeting (either as two orthree-dimensional images), or can be a three-dimensional image thatappears above, in the plane of, and below the sheeting. The images canbe in black and white or in color, and can appear to move with theobserver. The floating images can be observed by a viewer with theunaided eye.

The floating images of the microlens sheeting may be formed within alayer of microlensed material, without requiring an adjacent layer ofmaterial. The shape of the microlenses and the thickness of the layer ofmaterial on which the microlenses are formed are selected such thatcollimated light incident to the array is focused at regions within thelayer of the sheeting. The energy of the incident light impinging uponthe microlens sheeting is focused by the individual microlenses toregions within the sheeting. This focused energy modifies the layer toprovide an image, the size, shape, and appearance of which depends onthe interaction between the light rays and the microlenses. For example,light rays may form images associated with each of the microlenseswithin the layer at a damaged portion as a result of photodegradation,charring, or other damage to the sheeting.

The inventive sheeting having a composite image as described may be usedin a variety of applications such as securing tamperproof images insecurity documents, passports, identification cards, financialtransaction cards (e.g. credit cards), license plates, or otherarticles.

In one embodiment, a sheeting comprises a layer of material having asurface of microlenses that form one or more images at positionsinternal to the layer of material, wherein at least one of the images isa partially complete image, and each of the images is associated with adifferent one of the microlenses, and wherein the microlenses haverefractive surfaces that transmit light to positions within the layer ofmaterial to produce a composite image from the images formed within thelayer of material so that the composite image appears to float above thesheeting, float below the sheeting or float in the plane of thesheeting.

In another embodiment, a sheeting comprises a single layer of materialhaving microlenses formed on a first side and a retroreflective portionformed on a second side opposite the microlenses, wherein the layer ofmaterial includes one or more images formed between the microlenses andthe retroreflective portion, and wherein the microlenses produce acomposite image that appears to float above the sheeting, float belowthe sheeting or float in the plane of the sheeting.

In a further embodiment, a sheeting comprises a layer of material havinga surface of microlenses, a retroreflective layer; and aradiation-sensitive layer disposed between the layer of material and theretroreflective layer, wherein the radiation-sensitive layer includesone or more images formed between the layer of material and theretroreflective portion, and wherein the microlenses produce, from theimages of the radiation-sensitive layer, a composite image that appearsto float above the sheeting, float below the sheeting or float in theplane of the sheeting.

In yet another embodiment, a sheeting having first and second sidescomprises a first layer of material having a surface of microlenses, anda second layer of material having a surface of microlenses disposedproximate to the first layer, wherein one or more images are formedwithin the sheeting at locations between the microlenses of the firstlayer and the microlenses of the second layer, wherein at least one ofthe images is a partially complete image, wherein each image isassociated with one of a plurality of microlenses of the first layer,and wherein the microlenses have refractive surfaces that transmit lightto positions within the sheeting to produce a composite image from theimages that appears to float above the sheeting, float below thesheeting, or float in the plane of the sheeting, and wherein themicrolenses of the first layer and the microlenses of the second layerare aligned such that the composite image can be viewed from both thefirst side and the second side of the sheeting.

In yet another embodiment, a sheeting comprises a single layer having afirst set of microlenses formed on a first side and a second set ofmicrolenses formed on a second side opposite the first set ofmicrolenses, wherein the single layer includes one or more images formedinternally to the single layer, and wherein, from the images, the firstset of microlenses and the second set of microlenses produce a compositeimage that can be viewed from both the first side and the second side ofthe sheeting.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described herein with reference to the appendeddrawings, in which:

FIG. 1 is an enlarged cross sectional view of an “exposed lens”microlens sheeting;

FIG. 2 is an enlarged cross sectional view of an “embedded lens”microlens sheeting;

FIG. 3 is an enlarged cross sectional view of a microlens sheetingcomprising a plano-convex base sheet;

FIG. 4 is a graphical representation of divergent energy impinging on amicrolens sheeting constructed of microspheres;

FIG. 5 is a plan view of a section of a microlens sheeting depictingsample images recorded in the material layer adjacent individualmicrospheres, and further showing that the recorded images range fromcomplete replication to partial replication of the composite image;

FIG. 6 is an optical micrograph of a microlens sheeting with a radiationsensitive material layer made of aluminum film that has been imaged toprovide a composite image that appears to float above the sheeting inaccordance with the present invention;

FIG. 7 is an optical micrograph of a microlens sheeting with a radiationsensitive material layer made of aluminum film that has been imaged toprovide a composite image that appears to float below the sheeting inaccordance with the present invention;

FIG. 8 is a geometrical optical representation of the formation of acomposite image that appears to float above the microlens sheeting;

FIG. 9 is a schematic representation of a sheeting having a compositeimage that appears to float above the inventive sheeting when thesheeting is viewed in reflected light;

FIG. 10 is a schematic representation of a sheeting having a compositeimage that appears to float above the inventive sheeting when thesheeting is viewed in transmitted light;

FIG. 11 is a geometrical optical representation of the formation of acomposite image that when viewed will appear to float below themicrolens sheeting;

FIG. 12 is a schematic representation of a sheeting having a compositeimage that appears to float below the inventive sheeting when thesheeting is viewed in reflected light;

FIG. 13 is a schematic representation of a sheeting having a compositeimage that appears to float below the inventive sheeting when thesheeting is viewed in transmitted light;

FIG. 14 is a depiction of an optical train for creating the divergentenergy used to form the composite images of this invention;

FIG. 15 is a depiction of a second optical train for creating thedivergent energy used to form the composite images of this invention;and

FIG. 16 is a depiction of a third optical train for creating thedivergent energy used to form the composite images of this invention.

FIG. 17 is an enlarged cross sectional view of an example sheeting thatcontains a single layer of microlenses.

FIG. 18 is an enlarged cross sectional view of an example sheetinghaving an array of microlenses on a first side and a retroreflectiveportion on a second side.

FIG. 19A is a schematic representation of an example sheeting havingmicrolens arrays on both sides of the sheeting, and a composite imagethat appears to an observer on either side of the sheeting to floatabove the inventive sheeting.

FIG. 19B is a schematic representation of an example sheeting comprisinga first microlens layer, a second microlens layer, and a layer ofmaterial disposed between the first and second microlens layers.

FIG. 20 is an enlarged cross sectional view of an example sheetinghaving a microlens layer and a plurality of additional translucentlayers.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The microlens sheeting of the present invention provides a compositeimage, provided by individual images associated with a number of themicrolenses, that appears to be suspended, or to float, above, in theplane of, and/or below the sheeting.

To provide a complete description of the invention, microlens sheetingswill be described in Part I below, followed by descriptions of thematerial layers (preferably radiation sensitive material layers) of suchsheetings in Part II, radiation sources in Part III, and the imagingprocess in Part IV. Several examples are also provided to furtherexplain various embodiments of the present invention.

I. Microlens Sheeting

Microlens sheeting in which the images of this invention can be formedcomprise one or more discrete layers of microlenses with a layer ofmaterial (preferably a radiation-sensitive material or coating, asdescribed below) disposed adjacent to one side of the microlens layer orlayers. For example, FIG. 1 shows an “exposed lens” type of microlenssheeting 10 that includes a monolayer of transparent microspheres 12that are partially embedded in a binder layer 14, which is typically apolymeric material. The microspheres are transparent both to thewavelengths of radiation that may be used to image the layer ofmaterial, as well as to the wavelengths of light in which the compositeimage will be viewed. The layer of material 16 is disposed at the rearsurface of each microsphere, and in the illustrated embodiment typicallycontacts only a portion of the surface of each of the microspheres 12.This type of sheeting is described in greater detail in U.S. Pat. No.2,326,634 and is presently available from 3M under the designationScotchlite 8910 series reflective fabric.

FIG. 2 shows another suitable type of microlens sheeting. This microlenssheeting 20 is an “embedded-lens” type of sheeting in which themicrosphere lenses 22 are embedded in a transparent protective overcoat24, which is typically a polymeric material. The layer of material 26 isdisposed behind the microspheres at the back of a transparent spacerlayer 28, which is also typically a polymeric material. This type ofsheeting is described in greater detail in U.S. Pat. No. 3,801,183, andis presently available from 3M under the designation Scotchlite 3290series Engineer grade retroreflective sheeting. Another suitable type ofmicrolens sheeting is referred to as encapsulated lens sheeting, anexample of which is described in U.S. Pat. No. 5,064,272, and presentlyis available from 3M under the designation Scotchlite 3870 series HighIntensity grade retroreflective sheeting.

FIG. 3 shows yet another suitable type of microlens sheeting. Thissheeting comprises a transparent plano-convex or aspheric base sheet 30having first and second broad faces, the second face 32 beingsubstantially planer and the first face having an array of substantiallyhemi-spheroidal or hemi-aspheroidal microlenses 34. The shape of themicrolenses and thickness of the base sheet are selected such thatcollimated light incident to the array is focused approximately at thesecond face. The layer of material 36 is provided on the second face.Sheeting of this kind is described in, for example, U.S. Pat. No.5,254,390, and is presently available from 3M under the designation 2600series 3M Secure Card receptor.

The microlenses of the sheeting preferably have an image formingrefractive surface in order for image formation to occur; generally thisis provided by a curved microlens surface. For curved surfaces, themicrolens will preferably have a uniform index of refraction. Otheruseful materials that provide a gradient refractive index (GRIN) willnot necessarily need a curved surface to refract light. The microlenssurfaces are preferably spherical in nature, but aspherical surfaces arealso acceptable. The microlenses may have any symmetry, such ascylindrical or spherical, provided real images are formed by therefraction surfaces. The microlenses themselves can be of discrete form,such as round plano-convex lenslets, round double convex lenslets, rods,microspheres, beads, or cylindrical lenslets. Materials from which themicrolenses can be formed include glass, polymers, minerals, crystals,semiconductors and combinations of these and other materials.Non-discrete microlens elements may also be used. Thus, microlensesformed from a replication or embossing process (where the surface of thesheeting is altered in shape to produce a repetitive profile withimaging characteristics) can also be used.

Microlenses with a uniform refractive index of between 1.5 and 3.0 overthe visible and infrared wavelengths are most useful. Suitable microlensmaterials will have minimal absorption of visible light, and inembodiments in which an energy source is used to image aradiation-sensitive layer the materials should exhibit minimalabsorption of the energy source as well. The refractive power of themicrolens, whether the microlens is discrete or replicated, andregardless of the material from which the microlenses are made, ispreferably such that the light incident upon the refracting surface willrefract and focus on the opposite side of the microlens. Morespecifically, the light will be focused either on the back surface ofthe microlens or on the material adjacent to the microlens. Inembodiments in which the material layer is radiation sensitive, themicrolenses preferably form a demagnified real image at the appropriateposition on that layer. Demagnification of the image by approximately100 to 800 times is particularly useful for forming images that havegood resolution. The construction of the microlens sheeting to providethe necessary focusing conditions so that energy incident upon the frontsurface of the microlens sheeting is focused upon a material layer thatis preferably radiation sensitive is described in the U.S. patentsreferenced earlier in this section.

Microspheres with diameters ranging from 15 micrometers to 275micrometers are preferable, though other sized microspheres may be used.Good composite image resolution can be obtained by using microsphereshaving diameters in the smaller end of the aforementioned range forcomposite images that are to appear to be spaced apart from themicrosphere layer by a relatively short distance, and by using largermicrospheres for composite images that are to appear to be spaced apartfrom the microsphere layer by larger distances. Other microlens, such asplano-convex, cylindrical, spherical or aspherical microlenses havinglenslet dimensions comparable to those indicated for the microspheres,can be expected to produce similar optical results.

II. Layer of Material

As noted above, a layer of material is provided adjacent to themicrolenses. The layer of material may be highly reflective as in someof the microlens sheetings described above, or it may have lowreflectivity. When the material is highly reflective, the sheeting mayhave the property of retroreflectivity as described in U.S. Pat. No.2,326,634. Individual images formed in the material associated with aplurality of microlenses, when viewed by an observer under reflected ortransmitted light, provide a composite image that appears to besuspended, or float, above, in the plane of, and/or below the sheeting.Although other methods may be used, the preferred method for providingsuch images is to provide a radiation sensitive material as the materiallayer, and to use radiation to alter that material in a desired mannerto provide the image. Thus, although the invention is not limitedthereby, the remaining discussion of the layer of material adjacent themicrolenses will be provided largely in the context of a radiationsensitive material layer.

Radiation sensitive materials useful for this invention include coatingsand films of metallic, polymeric and semiconducting materials as well asmixtures of these. As used in reference to the present invention, amaterial is “radiation sensitive” if upon exposure to a given level ofvisible or other radiation the appearance of the material exposedchanges to provide a contrast with material that was not exposed to thatradiation. The image created thereby could thus be the result of acompositional change, a removal or ablation of the material, a phasechange, or a polymerization of the radiation sensitive coating. Examplesof some radiation sensitive metallic film materials include aluminum,silver, copper, gold, titanium, zinc, tin, chromium, vanadium, tantalum,and alloys of these metals. These metals typically provide a contrastdue to the difference between the native color of the metal and amodified color of the metal after exposure to the radiation. The image,as noted above, may also be provided by ablation, or by the radiationheating the material until an image is provided by optical modificationof the material. U.S. Pat. No. 4,743,526, for example, describes heatinga metal alloy to provide a color change.

In addition to metallic alloys, metallic oxides and metallic suboxidescan be used as a radiation sensitive medium. Materials in this classinclude oxide compounds formed from aluminum, iron, copper, tin andchromium. Non-metallic materials such as zinc sulfide, zinc selenide,silicon dioxide, indium tin oxide, zinc oxide, magnesium fluoride andsilicon can also provide a color or contrast that is useful for thisinvention.

Multiple layers of thin film materials can also be used to provideunique radiation sensitive materials. These multilayer materials can beconfigured to provide a contrast change by the appearance or removal ofa color or contrast agent. Exemplary constructions include opticalstacks or tuned cavities that are designed to be imaged (by a change incolor, for example) by specific wavelengths of radiation. One specificexample is described in U.S. Pat. No. 3,801,183, which discloses the useof cryolite/zinc sulphide (Na₃AlF₆/ZnS) as a dielectric mirror. Anotherexample is an optical stack composed of chromium/polymer (such as plasmapolymerized butadiene)/silicon dioxide/aluminum where the thickness ofthe layers are in the ranges of 4 nm for chromium, between 20 nm and 60nm for the polymer, between 20 nm and 60 nm for the silicon dioxide, andbetween 80 nm and 100 nm for the aluminum, and where the individuallayer thicknesses are selected to provide specific color reflectivity inthe visible spectrum. Thin film tuned cavities could be used with any ofthe single layer thin films previously discussed. For example, a tunedcavity with an approximately 4 nm thick layer of chromium and thesilicon dioxide layer of between about 100 nm and 300 nm, with thethickness of the silicon dioxide layer being adjusted to provide acolored imaged in response to specific wavelengths of radiation.

Radiation sensitive materials useful for this invention also includethermochromic materials. “Thermochromic” describes a material thatchanges color when exposed to a change in temperature. Examples ofthermochromic materials useful in this invention are described in U.S.Pat. No. 4,424,990, and include copper carbonate, copper nitrate withthiourea, and copper carbonate with sulfur containing compounds such asthiols, thioethers, sulfoxides, and sulfones. Examples of other suitablethermochromic compounds are described in U.S. Pat. No. 4,121,011,including hydrated sulfates and nitrides of boron, aluminum, andbismuth, and the oxides and hydrated oxides of boron, iron, andphosphorus.

Naturally, if the material layer is not going to be imaged using asource of radiation, then the material layer can, but is not requiredto, be radiation sensitive. Radiation sensitive materials are preferredfor ease of manufacturing, however, and thus a suitable radiation sourceis preferably also used.

III. Radiation Sources

As noted above, a preferred manner of providing the image patterns onthe layer of material adjacent the microlenses is to use a radiationsource to image a radiation sensitive material. Any energy sourceproviding radiation of the desired intensity and wavelength can be usedwith the method of the present invention. Devices capable of providingradiation having a wavelength of between 200 nm and 11 micrometers arebelieved to be particularly preferred. Examples of high peak powerradiation sources useful for this invention include excimer flashlamps,passively Q-switched microchip lasers, and Q-switched Neodymiumdoped-yttrium aluminum garnet (abbreviated Nd:YAG), Neodymiumdoped-yttrium lithium fluoride (abbreviated Nd:YLF) and Titaniumdoped-sapphire (abbreviated Ti: sapphire) lasers. These high peak powersources are most useful with radiation sensitive materials that formimages through ablation—the removal of material or in multiphotonabsorption processes. Other examples of useful radiation sources includedevices that give low peak power such as laser diodes, ion lasers, nonQ-switched solid state lasers, metal vapor lasers, gas lasers, arc lampsand high power incandescent light sources. These sources areparticularly useful when the radiation sensitive medium is imaged by anon-ablative method.

For all useful radiation sources, the energy from the radiation sourceis directed toward the microlens sheeting material and controlled togive a highly divergent beam of energy. For energy sources in theultraviolet, visible, and infrared portions of the electromagneticspectrum, the light is controlled by appropriate optical elements,examples of which are shown in FIGS. 14, 15, and 16 and described ingreater detail below. In one embodiment, a requirement of thisarrangement of optical elements, commonly referred to as an opticaltrain, is that the optical train direct light toward the sheetingmaterial with appropriate divergence or spread so as to irradiate themicrolens and thus the material layer at the desired angles. Thecomposite images of the present invention are preferably obtained byusing light spreading devices with numerical apertures (defined as thesine of the half angle of the maximum diverging rays) of greater than orequal to 0.3. Light spreading devices with larger numerical aperturesproduce composite images having a greater viewing angle, and a greaterrange of apparent movement of the image.

IV. Imaging Process

An exemplary imaging process according to this invention consists ofdirecting collimated light from a laser through a lens toward themicrolens sheeting. To create a sheeting having a floating image, asdescribed further below, the light is transmitted through a diverginglens with a high numerical aperture (NA) to produce a cone of highlydivergent light. A high NA lens is a lens with a NA equal to or greaterthan 0.3. The radiation sensitive coating side of the microspheres ispositioned away from the lens, so that the axis of the cone of light(the optical axis) is perpendicular to the plane of the microlenssheeting.

Because each individual microlens occupies a unique position relative tothe optical axis, the light impinging on each microlens will have aunique angle of incidence relative to the light incident on each othermicrolens. Thus, the light will be transmitted by each microlens to aunique position on the material layer, and produce a unique image. Moreprecisely, a single light pulse produces only a single imaged dot on thematerial layer, so to provide an image adjacent each microlens, multiplepulses of light are used to create that image out of multiple imageddots. For each pulse, the optical axis is located at a new positionrelative to the position of the optical axis during the previous pulse.These successive changes in the position of the optical axis relative tothe microlenses results in a corresponding change in the angle ofincidence upon each microlens, and accordingly in the position of theimaged dot created in the material layer by that pulse. As a result, theincident light focusing on the backside of the microsphere images aselected pattern in the radiation sensitive layer. Because the positionof each microsphere is unique relative to every optical axis, the imageformed in the radiation sensitive material for each microsphere will bedifferent from the image associated with every other microsphere.

Another method for forming floating composite images uses a lens arrayto produce the highly divergent light to image the microlensed material.The lens array consists of multiple small lenses all with high numericalapertures arranged in a planar geometry. When the array is illuminatedby a light source, the array will produce multiple cones of highlydivergent light, each individual cone being centered upon itscorresponding lens in the array. The physical dimensions of the arrayare chosen to accommodate the largest lateral size of a composite image.By virtue of the size of the array, the individual cones of energyformed by the lenslets will expose the microlensed material as if anindividual lens was positioned sequentially at all points of the arraywhile receiving pulses of light. The selection of which lenses receivethe incident light occurs by the use of a reflective mask. This maskwill have transparent areas corresponding to sections of the compositeimage that are to be exposed and reflective areas where the image shouldnot be exposed. Due to the lateral extent of the lens array, it is notnecessary to use multiple light pulses to trace out the image.

By having the mask fully illuminated by the incident energy, theportions of the mask that allow energy to pass through will form manyindividual cones of highly divergent light outlining the floating imageas if the image was traced out by a single lens. As a result, only asingle light pulse is needed to form the entire composite image in themicrolens sheeting. Alternatively, in place of a reflective mask, a beampositioning system, such as a galvometric xy scanner, can be used tolocally illuminate the lens array and trace the composite image on thearray. Since the energy is spatially localized with this technique, onlya few lenslets in the array are illuminated at any given time. Thoselenslets that are illuminated will provide the cones of highly diverginglight needed to expose the microlensed material to form the compositeimage in the sheetings.

The lens array itself can be fabricated from discrete lenslets or by anetching process to produce a monolithic array of lenses. Materialssuitable for the lenses are those that are non-absorbing at thewavelength of the incident energy. The individual lenses in the arraypreferably have numerical apertures greater than 0.3 and diametersgreater than 30 micrometers but less than 10 mm. These arrays may haveantireflection coatings to reduce the effects of back reflections thatmay cause internal damage to the lens material. In addition, singlelenses with an effective negative focal length and dimensions equivalentto the lens array may also be used to increase the divergence of thelight leaving the array. Shapes of the individual lenslets in amonolithic array are chosen to have a high numerical aperture andprovide a large fill factor of approximately greater than 60%.

FIG. 4 is a graphical schematic representation of divergent energyimpinging on a microlens sheeting. The portion of the material layer onor in which an image I is formed is different for each microlens,because each microlense “sees” the incoming energy from a differentperspective. Thus, a unique image is formed in the material layerassociated with each microlens.

After imaging, depending upon the size of the extended object, a full orpartial image of the object will be present in the radiation sensitivematerial behind each microsphere. The extent to which the actual objectis reproduced as an image behind a microsphere depends on the energydensity incident upon the microsphere. Portions of an extended objectmay be distant enough from a region of microlenses that the energyincident upon those microspheres has an energy density lower than thelevel of radiation required to modify that material. Moreover, for aspatially extended image, when imaging with a fixed NA lens, not allportions of the sheeting will be exposed to the incident radiation forall parts of the extended object. As a result, those portions of theobject will not be modified in the radiation sensitive medium and only apartial image of the object will appear behind the microspheres. FIG. 5is a perspective view of a section of a microlens sheeting depictingsample images formed in the radiation sensitive layer adjacent toindividual microspheres, and further showing that the recorded imagesrange from complete replication to partial replication of the compositeimage. FIGS. 6 and 7 are optical micrographs of a microlens sheetingimaged according to this invention, in which the radiation sensitivelayer is an aluminum layer. As shown therein some of the images arecomplete, and others are partial.

These composite images can also be thought of as the result of thesumming together of many images, both partial and complete, all withdifferent perspectives of a real object. The many unique images areformed through an array of miniature lenses, all of which “see” theobject or image from a different vantage point. Behind the individualminiature lenses, a perspective of the image is created in the materiallayer that depends on the shape of the image and the direction fromwhich the imaging energy source was received. However, not everythingthat the lens sees is recorded in the radiation sensitive material. Onlythat portion of the image or object seen by the lens that has sufficientenergy to modify the radiation sensitive material will be recorded.

The “object” to be imaged is formed through the use of an intense lightsource by either tracing the outline of the “object” or by the use of amask. For the image thus recorded to have a composite aspect, the lightfrom the object must radiate over a broad range of angles. When thelight radiating from an object is coming from a single point of theobject and is radiating over a broad range of angles, all the light raysare carrying information about the object, but only from that singlepoint, though the information is from the perspective of the angle ofthe light ray. Now consider that in order to have relatively completeinformation about the object, as carried by the light rays, light mustradiate over a broad range of angles from the collection of points thatconstitute the object. In this invention, the range of angles of thelight rays emanating from an object is controlled by optical elementsinterposed between the object and the microlens material. These opticalelements are chosen to give the optimum range of angles necessary toproduce a composite image. The best selection of optical elementsresults in a cone of light whereby the vertex of the cone terminates atthe position of the object. Optimum cone angles are greater than about40 degrees.

The object is demagnified by the miniature lenses and the light from theobject is focused onto the energy sensitive coating against the backsideof the miniature lens. The actual position of the focused spot or imageat the backside of the lens depends upon the direction of the incidentlight rays originating from the object. Each cone of light emanatingfrom a point on the object illuminates a fraction of the miniaturelenses and only those miniature lenses illuminated with sufficientenergy will record a permanent image of that point of the object.

Geometrical optics will be used to describe the formation of variouscomposite images according to the present invention. As notedpreviously, the imaging processes described below are preferred, but notexclusive, embodiments of the invention.

A. Creating a Composite Image that Floats Above the Sheeting

Referring to FIG. 8, incident energy 100 (light, in this example) isdirected onto a light diffuser 101 to homogenize any non-uniformities inthe light source. The diffusely scattered light 100 a is captured andcollimated by a light collimator 102 that directs the uniformlydistributed light 100 b towards a diverging lens 105 a. From thediverging lens, the light rays 100 c diverge toward the microlenssheeting 106.

The energy of the light rays impinging upon the microlens sheeting 106is focused by the individual microlenses 111 onto the material layer (aradiation sensitive coating 112, in the illustrated embodiment). Thisfocused energy modifies the radiation sensitive coating 112 to providean image, the size, shape, and appearance of which depends on theinteraction between the light rays and the radiation sensitive coating.

The arrangement shown in FIG. 8 would provide a sheeting having acomposite image that appears to an observer to float above the sheetingas described below, because diverging rays 100 c, if extended backwardthrough the lens, would intersect at the focal point 108 a of thediverging lens. Stated differently, if a hypothetical “image ray” weretraced from the material layer through each of the microspheres and backthrough the diverging lens, they would meet at 108 a, which is where thecomposite image appears.

B. Viewing a Composite Image that Floats Above the Sheeting

A sheeting that has a composite image may be viewed using light thatimpinges on the sheeting from the same side as the observer (reflectedlight), or from the opposite side of the sheeting as the observer(transmitted light), or both. FIG. 9 is a schematic representation of acomposite image that appears to the unaided eye of an observer A tofloat above the sheeting when viewed under reflected light. An unaidedeye may be corrected to normal vision, but is not otherwise assisted by,for example, magnification or a special viewer. When the imaged sheetingis illuminated by reflected light, which may be collimated or diffuse,light rays are reflected back from the imaged sheeting in a mannerdetermined by the material layer struck by the light rays. Bydefinition, the images formed in the material layer appear differentthan the non-imaged portions of the material layer, and thus an imagecan be perceived.

For example, light L1 may be reflected by the material layer back towardthe observer. However, the material layer may not reflect light L2 backtoward the observer well, or at all, from the imaged portions thereof.Thus, the observer may detect the absence of light rays at 108 a, thesummation of which creates a composite image that appears to float abovethe sheeting at 108 a. In short, light may be reflected from the entiresheeting except the imaged portions, which means that a relatively darkcomposite image will be apparent at 108 a.

It is also possible that the nonimaged material would absorb or transmitincident light, and that the imaged material would reflect or partiallyabsorb incident light, respectively, to provide the contrast effectrequired to provide a composite image. The composite image under thosecircumstances would appear as a relatively bright composite image incomparison to the remainder of the sheeting, which would appearrelatively dark. This composite image may be referred to as a “realimage” because it is actual light, and not the absence of light, thatcreates the image at focal point 108 a. Various combinations of thesepossibilities can be selected as desired.

Certain imaged sheetings can also be viewed by transmitted light, asshown in FIG. 10. For example, when the imaged portions of the materiallayer are translucent and the nonimaged portions are not, then mostlight L3 will be absorbed or reflected by the material layer, whiletransmitted light L4 will be passed through the imaged portions of thematerial layer and directed by the microlenses toward the focal point108 a. The composite image will be apparent at the focal point, where itwill in this example appear brighter than the remainder of the sheeting.This composite image may be referred to as a “real image” because it isactual light, and not the absence of light, that creates the image atfocal point 108 a.

Alternatively, if the imaged portions of the material layer are nottranslucent but the remainder of the material layer is, then the absenceof transmitted light in the areas of the images will provide a compositeimage that appears darker than the remainder of the sheeting.

C. Creating a Composite Image that Floats Below the Sheeting

A composite image may also be provided that appears to be suspended onthe opposite side of the sheeting from the observer. This floating imagethat floats below the sheeting can be created by using a converging lensinstead of the diverging lens 105 shown in FIG. 8. Referring to FIG. 11,the incident energy 100 (light, in this example) is directed onto adiffuser 101 to homogenize any non-uniformities in the light source. Thediffuse light 100 a is then collected and collimated in a collimator 102that directs the light 100 b toward a converging lens 105 b. From theconverging lens, the light rays 100 d are incident on the microlenssheeting 106, which is placed between the converging lens and the focalpoint 108 b of the converging lens.

The energy of the light rays impinging upon the microlens sheeting 106is focused by the individual microlenses 111 onto the material layer (aradiation sensitive coating 112, in the illustrated embodiment). Thisfocused energy modifies the radiation sensitive coating 112 to providean image, the size, shape, and appearance of which depends on theinteraction between the light rays and the radiation sensitive coating.The arrangement shown in FIG. 11 would provide a sheeting having acomposite image that appears to an observer to float below the sheetingas described below, because converging rays 100 d, if extended throughthe sheeting, would intersect at the focal point 108 b of the diverginglens. Stated differently, if a hypothetical “image ray” were traced fromthe converging lens 105 b through each of the microspheres and throughthe images in the material layer associated with each microlens, theywould meet at 108 b, which is where the composite image appears.

D. Viewing a Composite Image that Floats Below the Sheeting

Sheeting having a composite image that appears to float below thesheeting can also be viewed in reflected light, transmitted light, orboth. FIG. 12 is a schematic representation of a composite image thatappears to float below the sheeting when viewed under reflected light.For example, light L5 may be reflected by the material layer back towardthe observer. However, the material layer may not reflect light L6 backtoward the observer well, or at all, from the imaged portions thereof.Thus, the observer may detect the absence of light rays at 108 b, thesummation of which creates a composite image that appears to float belowthe sheeting at 108 b. In short, light may be reflected from the entiresheeting except the imaged portions, which means that a relatively darkcomposite image will be apparent at 108 b.

It is also possible that the nonimaged material would absorb or transmitincident light, and that the imaged material would reflect or partiallyabsorb incident light, respectively, to provide the contrast effectrequired to provide a composite image. The composite image under thosecircumstances would appear as a relatively bright composite image incomparison to the remainder of the sheeting, which would appearrelatively dark. Various combinations of these possibilities can beselected as desired.

Certain imaged sheetings can also be viewed by transmitted light, asshown in FIG. 13. For example, when the imaged portions of the materiallayer are translucent and the nonimaged portions are not, then mostlight L7 will be absorbed or reflected by the material layer, whiletransmitted light L8 will be passed through the imaged portions of thematerial layer. The extension of those rays, referred to herein as“image rays,” back in the direction of the incident light results in theformation of a composite image at 108 b. The composite image will beapparent at the focal point, where it will in this example appearbrighter than the remainder of the sheeting.

Alternatively, if the imaged portions of the material layer are nottranslucent but the remainder of the material layer is, then the absenceof transmitted light in the areas of the images will provide a compositeimage that appears darker than the remainder of the sheeting.

E. Complex Images

Composite images made in accordance with the principles of the presentinvention may appear to be either two-dimensional, meaning that theyhave a length and width, and appear either below, or in the plane of, orabove the sheeting, or three-dimensional, meaning that they have alength, width, and height. Three-dimensional composite images may appearbelow or above the sheeting only, or in any combination of below, in theplane of, and above the sheeting, as desired. The term “in the plane ofthe sheeting” refers only generally to the plane of the sheeting whenthe sheeting is laid flat. That is, sheeting that isn't flat can alsohave composite images that appear to be at least in part “in the planeof the sheeting” as that phrase is used herein.

Three-dimensional composite images do not appear at a single focalpoint, but rather as a composite of images having a continuum of focalpoints, with the focal points ranging from one side of the sheeting toor through the sheeting to a point on the other side. This is preferablyachieved by sequentially moving either the sheeting or the energy sourcerelative to the other (rather than by providing multiple differentlenses) so as to image the material layer at multiple focal points. Theresulting spatially complex image essentially consists of manyindividual dots. This image can have a spatial extent in any of thethree cartesian coordinates relative to the plane of the sheeting.

In another type of effect, a composite image can be made to move into aregion of the microlensed sheeting where it disappears. This type ofimage is fabricated in a fashion similar to the levitation examples withthe addition of placing an opaque mask in contact with the microlensedmaterials to partially block the imaging light for part of themicrolensed material. When viewing such an image, the image can be madeto move into the region where the imaging light was either reduced oreliminated by the contact mask. The image seems to “disappear” in thatregion.

The composite images formed according to the present invention can havevery wide viewing angles, meaning that an observer can see the compositeimage across a wide range of angles between the plane of the sheetingand the viewing axis. Composite images formed in microlens sheetingcomprised of a monolayer of glass microspheres having an averagediameter of approximately 70-80 micrometers and, when using an asphericlens with a numerical aperture of 0.64, are visible within a conicalfield of view whose central axis is determined by the optical axis ofthe incident energy. Under ambient lighting, the composite image soformed is viewable across a cone of about 80-90 degrees full angle.Utilizing an imaging lens with less divergence or lower NA can formsmaller half angle cones.

Images formed by the process of this invention can also be constructedthat have a restricted viewing angle. In other words, the image wouldonly be seen if viewed from a particular direction, or minor angularvariations of that direction. Such images are formed similar to themethod described in Example One below, except that light incident on thefinal aspheric lens is adjusted so that only a portion of the lens isilluminated by the laser radiation. The partial filling of the lens withincident energy results in a restricted cone of divergent light incidentupon the microlensed sheeting. For aluminum coated microlens sheeting,the composite image appears only within a restricted viewing cone as adark gray image on a light gray background. The image appears to befloating relative to the microlens sheeting.

EXAMPLES

This invention will be further explained by the following Examples,which may for convenience reference certain of the Figures.

Example One

This example describes an embedded lens sheeting with an aluminummaterial layer, and a composite image that appeared to float above thesheeting. An optical train of the type depicted in FIG. 14 was used toform the floating image. The optical train consisted of a SpectraPhysics Quanta-Ray™ DCR-2(10) Nd:YAG laser 300 operating in a Q-switchedmode at its fundamental wavelength of 1.06 micrometers. The pulse widthof this laser is typically from 10-30 ns. Following the laser, theenergy was redirected by a 99% reflective turning mirror 302, a groundglass diffuser 304, a 5× beam expansion telescope 306, and an asphericlens 308 with a numerical aperture of 0.64 and a focal length of 39.0mm. The light from the aspheric lens 308 was directed toward an XYZstage 310. The stage was composed of three linear stages, and isavailable from Aerotech Inc. of Pittsburgh, Pa. under the designationATS50060. One linear stage was used to move the aspheric lens along theaxis between the aspheric focal point and the microlens sheeting (thez-axis), and the other two stages enabled the sheeting to be moved intwo mutually orthogonal horizontal axes relative to the optical axis.

The laser light was directed toward the ground glass diffuser 304 toeliminate any beam inhomogeneities caused by thermal lensing.Immediately adjacent to the diffuser, a 5× beam expansion telescope 306collimated the diverging light from the diffuser and enlarged the lightbeam to fill the aspherical lens 308.

In this example, the aspheric lens was positioned above the XY plane ofthe XYZ stage so that the focal point of the lens was 1 cm above themicrolens sheeting 312. An apertured energy meter available from Gentec,Inc., of Saint-Fey, Quebec, Canada under the designation ED500 with amechanical mask, was used to control the energy density at the plane ofthe sheeting. The laser output was adjusted to obtain approximately 8milliJoules per square centimeter (8 mJ/cm²) over the illuminated areaof the energy meter 1 cm from the focal point of the aspheric lens. Asample of embedded lens sheeting 312 with an 80 nm thick aluminumradiation sensitive layer was affixed to the XYZ stage 310 so that thealuminum coated side faced away from the aspherical lens 308.

A controller available from Aerotech, Inc. of Pittsburgh, Pa. under thedesignation U21 provided the necessary control signals for movement ofthe XYZ stage 312 and control voltages for pulsing of the laser 300. Thestages were moved by importing a CAD file into the controller with thex-y-z coordinate information, movement commands, and laser firingcommands necessary to produce the image. An arbitrary complex compositeimage was formed by coordinating the movement of the X, Y and Z stageswith the pulsing of the laser to trace the image in space above themicrolensed material. The stage speed was adjusted to 50.8centimeters/minute for a laser pulse rate of 10 Hz. This formedcontinuous composite lines in the aluminum layer adjacent themicrolenses.

When the microlensed sheeting was viewed in ambient light, the imageswere dark gray against a light gray background. For a fixed 1 cm spacingbetween the focal point and the surface of the beaded sheeting, theresulting image was a planar composite image that appeared to floatapproximately 1 cm above the sheeting. Moreover, the composite imagedisplayed reasonably large movement in relation to an observer's viewingperspective, so an observer could easily view different aspects of thecomposite image depending upon the viewing angle.

Example Two

In this example, an exposed lens sheeting construction with atransparent mirror radiation sensitive layer was used to form acomposite image that appeared to float below the microlens sheeting. Theoptical train used in Example One was also used in this Example. Themicrolensed sheeting was positioned relative to the aspheric lens 308 sothat the lens was nearly in contact with the microlens sheeting. Thelaser output was adjusted to achieve approximately 14 mJ/cm² directlybeneath the aspheric lens. The exposed lens sheeting consisted ofpartially embedded microspheres as described in U.S. Pat. No. 3,801,183,with a zinc-sulfide (ZnS) dielectric mirror vapor deposited onto oneside of the microspheres. The thickness of the ZnS layer was nominally60 nm. As in Example One, the laser was operated at 10 Hz while thesheeting was moved at 50.8 cm/min, resulting in the formation ofcontinuous composite lines in the microlensed sheeting. A “globe”pattern (a circle with four inscribed arcs) was traced by the stagingsystem.

Under ambient lighting, the globe appeared as a dark image against awhite/yellow background. The dark composite image appeared to floatapproximately 39 mm below the sheeting. The location of the compositeimage corresponded to the location of the focal point of the asphericlens, which for this Example correlated to approximately 39 mm behindthe lens.

Example Three

This Example describes forming a composite image in an exposed lenssheeting with an aluminum radiation sensitive layer using a lens arrayin place of a single aspheric lens. An optical train of the typedepicted in FIG. 15 was used to form a floating composite image. Theoptical train consisted of a Q-switched laser 300, a 99% reflectivemirror 302, an optical diffuser 304, and a beam expansion telescope 306.These components of the optical train used in this example are identicalto those described in Example One. Also included in the optical train ofthis Example was a two-dimensional lens array 407, a reflective mask 409and a negative bi-concave lens 411. Areas of the reflective mask 409were transparent, to coincide with the areas of the microlensed material412 to be exposed to the laser radiation, while the remaining surface ofthe mask was opaque or reflective.

The lens array 407 consisted of a fused silica refractive microlensarray available from MEMS Optical, LLC of Huntsville, Ala. under thedesignation 3038. This closed packed spherical lens array was placedalmost in contact with a negative biconcave lens 411 having a diameterof 75 mm and focal length of negative 150 mm. Exposed lens sheeting 412with an 80 nm thick aluminum radiation sensitive layer was placed within25 mm of the negative bi-concave lens 411. The microlensed material wasplaced approximately 1 cm from the focal length of the combined opticalpath of the mircrolens array and the negative bi-concave lens. Theoutput from the laser was adjusted to produce approximately 4 mJ/cm² atthe surface of the exposed lens surface of the microlensed sheeting. Asingle laser pulse was activated to expose the entire image.

The resulting imaged microlensed sheeting, when viewed in ambient light,revealed images that appeared to float approximately 1 cm above thesheeting. The image appeared dark gray against a light gray background.

Example Four

In this Example, the diverging light source was obtained by reflectionfrom a scattering source. The scattering reflector consisted of aceramic bead approximately 5 mm in diameter. An optical train of thetype depicted in FIG. 16 was used in this Example. It consisted of aQ-switched Nd:YAG laser 500, similar to that described in Example One,followed by a telescope 502 which reduced the size of the incident laserbeam to a diameter of approximately 1 mm. The light was then impingedupon the ceramic bead 504 at an angle sufficiently deviated from normalso as to illuminate approximately one quarter of the hemisphere of theceramic bead 504 facing the microlens sheeting 512. This was confirmedby viewing the scattered radiation through an infrared camera.

The ceramic bead 504 was positioned above the XY stage 510 at a distanceof approximately 25 mm. The incident light from the laser was adjustedto be parallel to the sample stage. Embedded lens sheeting 512 with an80 nm aluminum radiation sensitive layer was affixed to an XY stage 510and a controller provided control signals to the stage and laser. Thelaser output was adjusted to obtain approximately 8 mJ/cm² at thesurface of the microlens sheeting. Illumination of the ceramic bead 504was adjusted to obtain the most uniform light exposure to the surface ofthe microlensed sheeting 512. The XY stage 510 was moved at 50.8cm/minute with the laser pulsing at 10 Hz. A complex image was tracedout with the stage while the microlensed sheeting was exposed to thescattered radiation from the ceramic reflector.

In ambient light, a composite image floated approximately 25 mm abovethe sheeting, and appeared dark gray against a light gray background.The image had large movement relative to the observer's viewingposition. Under transmitted light, a luminous composite image floatedapproximately 25 mm above the sheeting.

Example Five

In this example, the material layer of an embedded lens sheetingconsisted of multilayer optical stacks, tuned for specific colors in thevisible spectrum. On one face of the microlensed base sheet, thin filmlayers were deposited by vacuum evaporation and plasma polymerization toobtain a layer sequence consisting of chromium/plasma polymerizedbutadiene/silicon dioxide/aluminum, with the chromium layer beingadjacent to the embedded lens. The thicknesses of the individualmaterials were adjusted to obtain colors in the red, green, and blueportions of the visible spectrum. Table 1 provides the specificthicknesses of the individual materials prepared.

TABLE 1 Multilayer Construction Sample Cr (nm) PP (nm) SiO₂ (nm) Al (nm)Color 1 4 97 0 80 Blue 2 4 65 65 80 Light Blue 3 4 89 65 80 Green 4 4165 20 80 Red/Blue

The coated microlens base sheets were then laminated to a backing withthe multilayers in contact with the laminating material. The liner ofthe microlens sheeting was then removed to expose the front surface ofthe embedded lenses with colors given by the above table.

An optical train as described in Example One was used to image thesamples of this example. In this example, the focal point of the aspherewas positioned 1 cm above the microlens sheeting. The laser output wasadjusted to obtain an energy density of 5 mJ/cm² at the surface of themicrolens sheeting. The optical properties of the multilayer stacks weremodified in the regions irradiated. A globe pattern was traced toprovide images in the multilayer stacks in a manner similar to thatdescribed in Example One.

In ambient lighting, the irradiated regions appeared light yellow toorange in color against the background color of the microlensedsheeting. All composite images appeared to float above the sheeting andmove relative to the observer.

Example Six

This example describes a second type of multilayer tuned stack as theradiation sensitive layer for producing a colored composite image. Theoptical stacks were prepared on a microlensed base sheet consisting ofembedded lens sheeting. On one face of the microlensed base sheets, thinfilm layers were deposited by vacuum evaporation to obtain a layersequence consisting of chromium/cryolite/aluminum (Cr/Na₃AlF₆/A1),chromium/silicon dioxide/aluminum (Cr/SiO₂/A1), or chromium/magnesiumfluoride/aluminum (Cr/MgF₂/A1), as shown in Table 2, below. Thethicknesses of the dielectric materials, SiO₂, Na₃AlF₆ and MgF₂, wereadjusted to obtain a variety of colors in the visible spectrum. Table 2provides the specific thicknesses of the individual materials preparedin the various samples.

TABLE 2 Multilayer Construction Imaging Cr Na₃AlF₆ SiO₂ MgF₂ Al EnergyThickness Thickness Thickness Thickness Thickness Density Sample (nm)(nm) (nm) (nm) (nm) Color (mJ/cm²) A 4.8 200 0 0 83 Blue 12.7 B 4.2 0135 0 83 Deep Blue 8.6 C 4.2 0 0 259 83 Aquagreen 8.6 D 4.2 0 275 0 83Violet 7.5 E 4.2 0 160 0 83 Green 7.5 F 4.2 0 225 0 83 Orange-tan 7.5

The coated microlens base sheets were then laminated to a backing suchthat the multilayer was in contact with the laminating material. Theliner of the microlens sheeting was then removed to expose the frontsurface of the embedded lenses with colors given by the above table.

An optical train as described in Example One was used to image thesesamples. In this example, the position of the final aspheric lens waspositioned to be almost in contact with the sample to provide acomposite image that appeared to float below the sheeting. The laserenergy was adjusted to obtain an energy density that would permanentlyalter the optical properties of the respective multilayer stacks, asshown in Table 2. The alphanumeric characters “SAMPLE” were traced forthe image in this material in a manner similar to that described inExample One. In ambient lighting, the composite image appeared dark witha white/yellow outline against the background color of the microlensedsheeting. All composite images appeared to float approximately 39 mmbelow the sheeting and to move with respect to an observer viewing thesheeting.

Example Seven

In this example, a color composite image was formed in an embedded lenssheeting using a phase change alloy of 50 atomic percent Silver and 50atomic percent of Zinc (Ag₅₀Zn₅₀) and a tuned bilayer stack consistingof chromium and silicon dioxide as the radiation sensitive layer. Thephase change alloy was not ablated by the applied radiation, while thetuned bilayer enhances the spectral reflectance in the blue portion ofthe visible electromagnetic spectrum. The radiation sensitive layer wasdeposited onto the spacer layer of the enclosed lens sheeting in amanner similar to the procedure used to deposit the thin film layers ofthe multilayer stack unto the microlensed base sheet in Example Five.First, the chromium and silicon dioxide layers were vacuum depositedonto the polymeric spacer layer to thicknesses of 40 nm and 260 nm,respectively. Next, an 80 nm thick layer of Ag₅₀Zn₅₀ alloy was sputterdeposited onto the silicon dioxide layer. The samples were thenlaminated and stripped to expose the clear portion of the microlenssheeting.

The sheeting, when viewed under ambient (reflected) light, appeared tobe violet-blue. An optical train similar to Example One was used toimage the Ag₅₀Zn₅₀ radiation sensitive layer. In place of the Q-switchedlaser, a continuous wave Nd:YAG laser operating at a wavelength of 1.06um, was used as the energy source. The pulse width was controlled by theuse of an acousto-optic modulator in the optical train. The first orderdiffraction beam was sent through an optical train of the type depictedin FIG. 14. Samples of the enclosed lens sheeting were affixed to an XYZstage. The laser power into the acousto-optic modulator was adjusted togive 810 mW of power at the microlensed material. The acousto-opticmodulator was set to achieve 20 Hz pulsing at 100 microsecond pulsewidths. A positive aspheric lens, as described in Example One, wasplaced 12 mm above the surface of the microlensed material. An image wastraced out with the XYZ stage while the laser radiation exposed theradiation sensitive layer.

When the sheeting was viewed in ambient lighting, the imaged regionsappeared light blue in color and floated about 12 mm above the microlenssheeting.

Example Eight

In this Example, a replicated lens structure with a copper radiationsensitive layer was used as the microlens sheeting. Replicated sheetingof the type described in U.S. Pat. No. 5,254,390 was used as themicrolens sheeting. A radiation sensitive layer of copper was vacuumevaporated on to the flat surface of the sheeting to a thickness of 80nm. The microreplicated microlensed material was exposed to laserradiation from an optical train as described in Example One. The finalaspheric lens was positioned with the focal point 6.5 mm away from thesurface of the microlensed material. The laser output was adjusted togive approximately 7 mJ/cm² at the surface of the sheeting. The laserwas set to pulse at 10 Hz while the XYZ staging moved at a speed of 50.8cm/minute. A “globe” pattern (a circle with four inscribed arcs) wastraced above the sample.

When the sheeting was viewed in ambient lighting, a whitish image of afloating globe could be seen against the copperish color of theradiation sensitive layer. This composite image appeared to float about6 mm above the sheeting.

Example Nine

This Example describes the combination of a planar composite image witha composite image that appeared to float below the sheeting. Exposedlens microlens sheeting with an 80 nm thick aluminum radiation sensitivelayer was imaged using the optical configuration described in ExampleOne. The aspheric lens was positioned nearly in contact with themicrolens sheeting, and the laser output was adjusted to yield 4 mJ/cm²at the sample surface. The controller was programmed to trace thealphanumeric characters “SAMPLE.” An absorptive mask was placed on topof the open sheeting. This mask was made by printing rows of thealphanumeric characters “3M” onto transparency sheets by way of aconventional photocopier. The alphanumeric characters absorbed theradiation while the surrounding areas would transmit the laserradiation. The exposed lens sheeting with this absorptive mask waspositioned so that the “SAMPLE” characters were formed over the top ofthe mask position.

When viewed under ambient lighting, the characters “SAMPLE” appeared tofloat about 39 mm below the sheeting, while the unexposed characters“3M” appeared to be in the plane of the sheeting. The “3M” characterswere only observable against the dark characters from the “SAMPLE”characters.

Example Ten

This Example describes a sheeting with a complex, three-dimensionalimage. An embedded lens microlens sheeting with an 80 nm thick aluminumradiation sensitive layer was used in this Example. The optical trainused in Example One was used. The microlensed sheeting was attached tothe XY plane of an XYZ translation stage, while an aspheric lens wasattached to the z-axis. The aspheric lens had a NA of 0.64 and a focallength of 39 mm. The controller was programmed to trace the outline ofan isometric cube with 5 cm long cube diagonals (the distance betweentwo opposite corners of the cube). The relative position and orientationof the cube as programmed in the controller placed one end of thecomposite cube image approximately 5 mm above the surface of thesheeting, and the other end of the composite cube image 5.5 cm abovethat surface. The cube image was oriented to place a corner of the cubeclosest to the observer.

During the tracing of the isometric cube, the energy per pulse from thelaser was controlled to yield a constant energy density of 8 mJ/cm² atthe sample surface regardless of the spacing between the diverging lensand the sheeting. The laser operated at 10 Hz and X, Y and Z stagesmoved at a speed of 50.8 cm/minute. The image of the isometric cube wascontinuously traced out in space above the microlensed sheeting by thecontroller.

When viewed in ambient lighting, the isometric composite cube imageappeared dark gray against a light gray background, floating frombetween 5 mm and 5.5 cm above the surface. Furthermore, as an observerchanged his or her viewing perspective, the isometric cube appeared torotate in space above the microlens sheeting to expose sides of the cubethat were previously obscured at different viewing angles.

Example Eleven

This Example describes a floating image that can be made to disappear.That is, the composite image can, by changing the viewing angle, be madeto disappear from or reappear to view. An embedded lens sheeting with an80 nm thick aluminum radiation sensitive layer was used. An opticaltrain similar to that in Example One was used to form the images, andthe distance of the aspheric lens from the sheeting was adjusted toposition the focal point 1 cm above the microlensed sheeting. Thecontroller was programmed to produce a “globe” pattern (a circle withfour inscribed arcs) and the laser output was adjusted to provide 8mJ/cm² at the sample surface. On the sample itself, a square section oftranslucent tape was attached to the surface of the embedded lenssheeting. The square section of tape was positioned so that during theimaging of the globe, a portion of the area imaged by the laser wouldoverlap the section covered by the translucent tape.

When the imaged sheeting was viewed under ambient light, a floatingglobe pattern was observed as a dark gray image against a light graybackground, floating 1 cm above the sheeting. By varying the viewingangle, the “globe” moved into or out of the region that was masked bythe translucent tape. When the globe moved into the masked region, theportion of the globe in that region disappears. When the globe moved outof the masked region, the portion of the globe in that regionreappeared. The composite image did not merely fade gradually away as itpassed into the masked region, but rather completely disappeared exactlywhen it passed into that region.

Imaged sheeting containing the composite images of this invention aredistinctive and impossible to duplicate with ordinary equipment. Thecomposite images can be formed in sheeting that is specificallydedicated to applications such as passports, identification badges,banknotes, identification graphics, and affinity cards. Documentsrequiring verification can have these images formed on the laminatedsheeting for identification, authenticity, and enhancement. Conventionalbonding means such as lamination, with or without adhesives, may beused. Providers of items of value, such as boxed electronic products,compact discs, driver's licenses, title documents, passports or brandedproducts, may simply apply the multilayer film of this invention totheir products and instruct their customers only to accept as authenticitems of value so labeled. For products requiring these protections,their appeal may be enhanced by the inclusion of sheeting containingcomposite images into their construction or by adhering such sheeting tothe products. The composite images may be used as display materials foradvertising, for license plates, and for numerous other applications inwhich the visual depiction of a unique image is desirable. Advertisingor information on large objects, such as signs, billboards, orsemitrailers, would draw increased attention when the composite imageswere included as part of the design.

Sheeting with the composite images has a very striking visual effect,whether in ambient light, transmitted light, or retroreflected light inthe case of retroreflective sheeting. This visual effect can be used asa decoration to enhance the appearance of articles to which the imagedsheeting is attached. Such an attachment could convey a heightened senseof fashion or style and could present a designer logo or brand in a verydramatic way. Envisioned uses of the sheeting for decoration includeapplications to apparel, such as everyday clothing, sports clothing,designer clothing, outerwear, footwear, caps, hats, gloves and the like.Similarly, fashion accessories could utilize imaged sheeting fordecoration, appearance, or brand identity. Such accessories couldinclude purses, wallets, briefcases, backpacks, fanny packs, computercases, luggage, notebooks and the like. Further decorative uses of theimaged sheeting could extend to a variety of objects that are commonlyembellished with a decorative image, brand, or logo. Examples includebooks, appliances, electronics, hardware, vehicles, sports equipment,collectibles, objects of art and the like.

When the decorative imaged sheeting is retroreflective, fashion or brandawareness can be combined with safety and personal protection.Retroreflective attachments to apparel and accessories are well knownand enhance the visibility and conspicuity of the wearer in low-lightconditions. When such retroreflective attachments incorporate thecomposite imaged sheeting, a striking visual effect can be achieved inambient, transmitted, or retroreflected light. Envisioned applicationsin the area of safety and protective apparel and accessories includeoccupational safety apparel, such as vests, uniforms, firefighter'sapparel, footwear, belts and hardhats; sports equipment and clothing,such as running gear, footwear, life jackets, protective helmets, anduniforms; safety clothing for children; and the like.

Attachment of the imaged sheeting to the aforementioned articles can beaccomplished by well known techniques, as taught in U.S. Pat. Nos.5,691,846 (Benson, Jr. et al.), 5,738,746 (Billingsley et al.),5,770,124 (Marecki et al.), and 5,837,347 (Marecki), the choice of whichdepends on the nature of the substrate material. In the case of a fabricsubstrate, the sheeting could be die cut or plotter cut and attached bysewing, hot-melt adhesive, mechanical fasteners, radio frequency weldingor ultrasonic welding. In the case of hardgoods, a pressure-sensitiveadhesive may be a preferred attachment technique.

In some cases, the image may be best formed after the sheeting isattached to a substrate or article. This would be especially useful whena custom or unique image was desired. For example, artwork, drawings,abstract designs, photographs, or the like could be computer generatedor digitally transferred to a computer and imaged on the sheeting, theunimaged sheeting having been previously attached to the substrate orarticle. The computer would then direct the image generation equipmentas described above. Multiple composite images may be formed on the samesheeting, and those composite images may be the same or different.Composite images may also be used along with other conventional imagessuch as printed images, holograms, isograms, diffraction gratings,kinegrams, photographs, and the like. The image may be formed in thesheeting before or after the sheeting is applied to an article orobject.

Translucent and Transparent Laminates

In certain embodiments, a sheeting may utilize one or more layers oftranslucent or transparent laminates as materials or combinations ofmaterials into which a floating image may be formed. For convenience,the invention will be described with respect to translucent materials;however, a range of materials may be used for the sheeting, includingtranslucent materials, semi-translucent materials, and transparentmaterials. The sheeting may form a construction that maintains acomplete or semi-translucent property, i.e., that allows light to passthrough the construction to some extent.

Translucent laminates may be combined with other functional materials.For example, a finished construction may be adhesively or mechanicallyapplied to an article. The overall combined article may be translucent,opaque, or a combination thereof. Translucent laminates may beconstructed from a variety of single- or multi-layer materials orcombinations of these materials. For example, such materials may includedyed or pigmented colored films, multilayer optical films, andinterference films. Such a translucent laminate may include a singlelayer of clear, dyed, or pigmented polyethylene terephthalate (PET),silicone, acrylate, polyurethane or other such material, with a layer ofradiation sensitive material, disposed adjacent the first layer, intowhich an image is formed. Another example is a layer of material, havingoptical elements (e.g., lenses) formed on a surface of the layer, ontowhich a second material is transferred by a laser material transferprocess or other printing-like process.

In some embodiments, the floating images of the invention may be formedwithin a single layer of a translucent laminate itself, formed due tomicrolenses on a surface of the single layer without requiring anadjacent layer of material. FIG. 17 is an enlarged cross sectional viewof a sheeting 600 that contains a single layer 630 of material havingmicrolenses 602 formed on a surface thereof. That is, layer 630 may beformed as a single layer of material, having a surface of microlenses,and may have a thickness sufficient to be self-supporting, making anadditional substrate unnecessary.

In the illustrated embodiment of FIG. 17, sheeting 600 comprises atransparent plano-convex or aspheric sheeting having first and secondsides, the second side 604 being substantially planar and the first sidehaving an array of substantially hemi-spheroidal or hemi-aspheroidalmicrolenses 602 formed thereon. The shape of the microlenses 602 and thethickness of the layer 630 are selected such that collimated light 608incident to the array is focused at regions 610 within the single layer630. The thickness of layer 630 depends at least in part on thecharacteristics of the microlenses 602, such as the distance at whichthe microlenses focus light. For example, microlenses may be used thatfocus light at a distance of 60 μm from the front of the lens. In someembodiments, the thickness of layer 630 may be between 20-100 μm.Microlenses 602 may be formed of clear or colored PET, silicone,acrylate, polyurethane, polypropylene or other material, by a processsuch as embossing or microreplication.

Incident energy, such as light 608 from energy source 606, is directedtowards sheeting 600. The energy of the light rays impinging uponsheeting 600 is focused by the individual microlenses 602 to regions 610within layer 630. This focused energy modifies layer 630 at regions 610to provide an image, the size, shape, and appearance of which depends onthe interaction between the light rays 608 and microlenses 602. Forexample, light rays 608 may form respective partial images, associatedwith each of the microlenses, at respective damage sites within layer630 as a result of photodegradation, charring, or other localized damageto layer 630. Regions 610 may in some examples be referred to as“photodegradation portions.” The individual images may be formed ofblack lines caused by the damage. The individual images formed in thematerial, when viewed by an observer under reflected or transmittedlight, provide a composite image that appears to be suspended, or float,above, in the plane of, and/or below the sheeting.

A radiation source, as described above with respect to Part III, may beused to form the individual images at regions 610 within layer 630 ofsheeting 600. For example, a high peak power radiation source may beused. One example of a radiation source that may be used to image thesheeting is a regeneratively amplified titanium:sapphire laser. Forexample, a titanium:sapphire laser operating at a wavelength of 800 nmwith a pulse duration of approximately 150 femtoseconds and a pulse rateof 250 Hz may be used to form the images within the sheeting.

In some embodiments, the described sheeting may possess opticalmicrostructures on both sides. FIG. 18 is an enlarged cross sectionalview of an exemplary sheeting 700 having an array of substantiallyhemi-spheroidal or hemi-aspheroidal microlenses 702 on a first side anda retroreflective portion 704 on a second side. As shown in FIG. 18,retroreflective portion 704 may be an array of corner cubes. However,other types of retroreflective surfaces or non-retroreflective opticalstructures may be formed on the surface of the second side of sheeting700 opposite microlenses 702.

For example, the second side of sheeting 700 may contain diffractiveelements, e.g., a diffractive grating, to provide a color-shiftingcapability or other optical functions. As another example, the secondside may be comprised of partial corner cubes, lenticular lens arrays,additional lenslet arrays, compound optical layers, or other opticalelements formed within the surface of the second side of sheeting 700.Moreover, the optical microstructures on the second side of sheeting 700may be uniform or variable in location, period, dimension, or angle, toprovide a variety of optical effects. The optical microstructures mayalso be coated with a semi-transparent layer of metal. As a result ofthese variations, sheeting 700 may provide an image on a color-shiftingbackground or may provide added optical functionality.

In another embodiment, microlenses 702 may be formed within only aportion of the first side of sheeting 700, while retroreflective portion704 covers substantially all of the second side of sheeting 700. In thismanner, an observer viewing sheeting 700 from the first side may seeboth a floating image and areas that appear retroreflective. Sheeting700 could be used as a security feature by checking retroreflectivity ofthe sheeting. In certain embodiments, retroreflective portion 704 maycontain corner cubes, and corners of those corner cubes may be bent soas to give a “sparkly” appearance in the portion not covered bymicrolenses 702.

Individual images associated with each of the plurality of microlenses702 may be formed within sheeting 700 as described above with respect toFIG. 17. In one embodiment, sheeting 700 may be a two-sidedmicrostructure in which microlenses 702 and retroreflective portion 704are constructed on opposite surfaces a single layer of material. Inanother embodiment, microlenses 702 and retroreflective portion 704 maybe two separate layers of material affixed together, such as bylamination. In this case, the individual images may be formed atlocations between the layer associated with microlenses 702 and thelayer associated with retroreflective portion 704. Alternatively, alayer of radiation-sensitive material may exist between the layerassociated with microlenses 702 and the layer associated withretroreflective portion 704, on which the individual images are formed.

A two-sided, single-layer sheeting with microstructures on both sidesand having a composite image may be viewed under reflected light ortransmitted light, or both. FIG. 19A is a schematic representation of asheeting 800 having a first side 802 and a second side 804, each of thefirst and second sides having an array of substantially hemi-spheroidalor hemi-aspheroidal microlenses. Sheeting 800 presents composite images806A and 806B (“composite images 806”) based on the viewing position ofan observer. For example, composite images 806A, 806B appear to anobserver A on the first side of sheeting 800 and an observer B on thesecond side of sheeting 800, respectively, to float above (i.e., infront of) the sheeting 800 when viewed under reflected light. Compositeimages 806 are formed by the sum of individual images formed in sheeting800 in a manner similar to that described above with respect to imagesformed within a layer of material adjacent a layer of microlenses.

Individual images may be formed at regions 805 in sheeting 800. Forexample, individual images may be formed as above by incident energyfrom an energy source that modifies sheeting 800 at regions 805. Each ofregions 805 may correspond to a respective microlens formed on firstside 802, or to a respective microlens formed on second side 804, orboth. In one embodiment, the microlenses formed on first side 802 may beselected to focus light rays incident to first side 802 to a region 805substantially in the middle of sheeting 800. As a result, compositeimages 806 produced by the individual images formed at regions 805 maybe viewed by observer A on the first side 802 of sheeting 800, or byobserver B on the second side 804 of sheeting 800. In one embodiment,the microlenses formed on first side 802 and second side 804 line uplaterally and are substantially equal in terms of thickness and focallength so as to allow the composite image within sheeting 800 to bevisible from either side of the sheeting 800.

The composite image 806A seen by observer A may be different in someways from the composite image 806B seen by observer B. For example,where the composite images 806 include features having visual depth, theapparent depth of the features may be reversed. In other words, featuresappearing closest to observer A may appear farthest to observer B.Although not illustrated, in other embodiments a composite image formedby individual images at regions 805 may float in the plane of thesheeting, below the sheeting, and/or be viewable under transmittedlight.

FIG. 19B is a schematic representation of a multi-layer sheeting 808comprising a first layer 810 having microlenses formed on a surfacethereof, a second layer 812 similarly having microlenses formed on asurface thereof, and a layer of material 816 disposed between the firstand second microlens layers. The outer surfaces of layers 810, 812 mayinclude an array of substantially hemi-spheroidal or hemi-aspheroidalmicrolenses. Layer of material 816 may be a transparent material.

As described above with respect to FIG. 19A, sheeting 808 presentscomposite images 814A and 814B (“composite images 814”). Compositeimages 814 appear to an observer A on the first side of sheeting 808 andan observer B on the second side of sheeting 808, respectively, to floatabove the sheeting 808 when viewed under reflected light. Compositeimages 814 are produced by the sum of individual images formed in thelayer of material 816, as described above. Layer of material 816 may bea radiation sensitive material as described above in Part II. As anotherexample, layer of material 816 may be a transparent laser-markablematerial, such as a doped polycarbonate layer on which a laser beamforms black marks. In one embodiment, layers 810, 812 may be attached bylamination. Layer of material 816 may comprise coatings, films, or othertypes of layers. For example, layer of material 816 may be a metallicspacer, a dielectric spacer, a corner-cube spacer, a diffraction gratingspacer, a multilayer optical film (MOF), or a compound optical spacer.Multiple layers of material of different kinds or colors may be providedin place of layer of material 816. In some embodiments, different imagesmay be formed within layer of material 816 from each side, and as aresult, different floating images may be visible to observers A and B.In another embodiment, images may be formed at regions within one offirst layer 810 and second layer 812.

FIGS. 19A and 19B illustrate sheetings having a composite image thatappears to an observer on either side of the sheeting to float above thesheeting. In some embodiments, the sheeting may provide atwo-dimensional or three-dimensional composite image that appears onboth sides of the sheeting. Such a sheeting may find application as anenhanced security feature, and also provide brand enhancement, brandauthentication, and eye-catching appeal.

FIG. 20 is an enlarged cross-sectional view of a sheeting 900 includinga layer 902 having microlenses formed in a surface thereof and aplurality of additional translucent layers 904A-904N (“translucentlayers 904”). Layer 902 may be substantially similar to layer 630 ofFIG. 17. That is, as described above, layer 902 may constitute a singlelayer of sufficient thickness so that individual images may be formedwithin layer 902. Additional translucent layers 904 may be added tosheeting 900 to produce added visual appearance (e.g., color, contrast,color-shift) and function. Translucent layers 904 may be layers havingoptical structures, e.g., lenses, corner cubes, lenticular lens arrays,positioned within the optical stack to add effects such as colorshifting and function. For example, a diffractive grating may add colorshifting effects, while lenses may provide imaging functionality.Sheeting 900 may be used to provide a high contrast white floating imageon a continuously variable color background. The individual imagesformed in the material, when viewed by an observer under reflected ortransmitted light, provide a composite image that appears to besuspended, or float, above, in the plane of, and/or below the sheeting.

As discussed above, a number of configurations are possible fortranslucent microlens sheetings. For example, a sheeting may include aspacer that results in images misaligned with respect to the lens array.This may produce movement of the image orthogonal to the movement of theobserver relative to the substrate. As another example, a single layerof microlenses may be formed from energy-absorption-appropriatematerials. A protective topcoat may be added to a sheeting to adddurability. Such a topcoat may be colored or transparent, and mayenhance image appearance and provide a mechanism with which to produce auniform background color. The layer having microlenses on a surface orthe additional translucent layers may be dyed or pigmented. The pigmentcolors may be customized.

The sheeting may provide an enhanced contrast floating image on asemi-transparent substrate, or a translucent color image on atranslucent substrate. The sheeting may provide multi-sided colorshifting floating images with tunable color shifting and tunable opticaleffects as a function of viewing angle or incident lighting angle. Thesheeting may provide the ability to selectively form images withinsubstrates via wavelengths. The microreplicated optical structures of asheeting may be band-pass microreplicated optical structures, such ascolored glass band-pass or interference band-pass microreplicatedoptical structures. Such structures may be capable of single ormulti-wavelength image formation, or may allow for secure imageformation with unique wavelengths. Creating a band-pass substrate mayprovide both security and visual utility. Security value may be added byincreasing the number of laser systems needed to reproduce amulti-colored floating image.

The microlens sheeting may be an “embedded-lens” type of sheeting inwhich the microsphere lenses are embedded in a transparent protectiveovercoat, which is typically a polymeric material. Clear or coloredglass or polymer beads may be substituted for the microreplicated lensoptics in the embodiments described above. For example, beads may bebonded on multilayer optical films (MOF) on both sides, with the MOF andbead size additionally being varied. As another example, beads may bebonded on a dielectric spacer on both sides. The beads may be bonded toboth sides of a diffraction grating spacer, with the diffraction gratingblaze and periodic structure being varied. The beads may be metal-coatedbeads bonded to both sides of a diffraction grating spacer. The gratingmay be varied from 2D to 3D grating. Periodic structures may be added tothe gratings to influence diffractive orders, viewing angles, and thelike. The above features may also be selectively combined to achieve asheeting having a desired effect.

The translucent laminates described above may be incorporated in backlitapplications, or may be applied in constructions that incorporatecolored, white, or variable lighting elements, variable intensitylighting, light guides, fiber-delivered light, color filters,fluorescent or phosphorescent materials. These lighting conditions maybe designed to change the appearance of the image or the overallsubstrate in time, via user interaction, or via environmentalconditions. In this manner, the construction provides a dynamicallyvarying floating image with variably visible information content.

The single and multi-layer sheetings using translucent layers, asdescribed above, may be used in a number of applications, includingsecurity documents and consumer decorative applications. For example,the floating image of the sheeting may be used for a floating watermarkas a translucent overlay, providing a secure feature through whichprinted information is visible. The sheeting may be made very thin (<1mm), which may enable integration of the sheeting into securitydocuments, passports, drivers licenses, currency, banknotes,identification cards, titles, personnel badges, proofs of purchase,authenticity certificates, corporate cards, financial transaction cards(e.g. credit cards), certificates, brand and asset protection labels,registration tags, tax stamps, gaming chips, license plates, validationstickers, or other items.

The sheetings may also be incorporated into materials used by creativedesigners. As another example, the sheeting may be incorporated intocomputer cases, keyboards, numeric key pads, or computer displays.

Example Twelve

The following example describes results obtained from a series ofexperiments. In this example, an image was formed in a microreplicatedacrylate lens sheeting, similar to the sheeting depicted in FIG. 17. Asolution was cast at a thickness of 75 microns on a 125-micron thickkapton film containing a microlens pattern. The solution was composed of75% by weight Photomer 6210 (aliphatic urethane acrylate, CognisCorporation, Cincinnati, Ohio)/25% by weight 1,6-hexanediol diacrylate(UCB Chemicals, Smyrna, Ga.) containing 1% TPO-L photoinitiator (BASFCorporation, Florham Park, N.J.). The coated film was cured by exposurein a nitrogen-purged environment to the output of a Fusion D lamp. Themicrolens pattern was composed of 30-micron diameter lenses in ahexagonal pattern with a lens spacing of 34 microns. Each microlens hadan aspheric shape described by a radius of curvature of 18.7 microns anda conic constant of −0.745. Measurements indicated that thesemicrolenses had an effective focal length of approximately 60 microns ata wavelength of 800 nm.

Virtual images were drawn in this 75-micron thick acrylate sheet byexposing the sheet to the output of a regeneratively amplifiedtitanium:sapphire laser (Spectra-Physics Hurricane) operating at awavelength of 800 nm with a pulse duration of approximately 150femtoseconds and a pulse rate of 250 Hz. The laser beam used to draw theimage had an average power in the range of 10-100 mW, depending on theimage float height, and was focused by an aspheric lens operating at anf-number of approximately 1.

These conditions resulted in virtual images composed of black lines,with float heights of 1-10 mm above the surface, on the clear, colorlessbackground provided by the acrylate sheeting. Cross-sectionalmicrographs of the sheeting indicated that the black lines in thevirtual images were formed by the integration of microimages behind theappropriate microlenses. Each microimage appeared to be composed ofpatterns of black material formed in the acrylate sheeting where thelaser light focused by the microlenses reached intensities higher thanthe breakdown intensity of the polymer (˜10¹⁷ W/m²).

Various modifications and combinations of the embodiments disclosed willbe apparent to those skilled in the art, and those modifications areintended to be within the scope of the invention as defined in theappended claims.

1. A laser system for forming a composite image on a microlens sheeting,comprising: a laser; a diverging lens; and a sheeting having a layer ofmaterial having a surface of microlenses, wherein each of themicrolenses occupies a different position on the layer of material;wherein the laser images the layer of material through the diverginglens, wherein each of the microlenses transmits the laser energy to aunique position on the material to form a respective image adjacent themicrolens.
 2. The laser system of claim 1, wherein the diverging lensincludes a focal point set in front of the microlenses opposite thelayer of material of the sheeting.
 3. The laser system of claim 1,further comprising a light collimator, wherein the laser images thelayer of material through the light collimator and diverging lens. 4.The laser system of claim 1, wherein the sheeting includes a compositeimage from the images formed within the material that appear to floatabove the sheeting, float below the sheeting or float in the plane ofthe sheeting.
 5. The laser system of claim 1, further comprising acontroller for movement of the microlens sheeting relative to the laser.6. The laser system of claim 1, wherein the laser provides multiplepatterned exposures with the sheeting being at a different positionrelative to the laser during each exposure.
 7. The laser system of claim1, wherein the sheeting is included in a security document.
 8. A lasersystem for forming a composite image on a microlens sheeting,comprising: a laser; a converging lens; and a sheeting having a layer ofmaterial having a surface of microlenses, wherein each of themicrolenses occupies a different position on the layer of material;wherein the laser images the layer of material through the converginglens, wherein each of the microlenses transmits the laser energy to aunique position on the material to form a respective image adjacent themicrolens.
 9. The laser system of claim 8, wherein the diverging lensincludes a focal point set in back of the sheeting opposite themicrolenses of the sheeting.
 10. The laser system of claim 8, furthercomprising a light collimator, wherein the laser images the layer ofmaterial through the light collimator and diverging lens.
 11. The lasersystem of claim 8, wherein the sheeting includes a composite image fromthe images formed within the material that appear to float above thesheeting, float below the sheeting or float in the plane of thesheeting.
 12. The laser system of claim 8, further comprising acontroller for movement of the microlens sheeting relative to the laser.13. The laser system of claim 8, wherein the laser provides multiplepatterned exposures with the sheeting being at a different positionrelative to the laser during each exposure.
 14. The laser system ofclaim 8, wherein the sheeting is included in a security document.